Hypothesis: Early Ketone Rise Predicts Tissue-Specific Autophagy via HDAC Inhibition Modulated by Genetic Variants

Core idea The speed at which β‑hydroxybutyrate (BHB) rises during an early fasting window predicts the magnitude of autophagy activation in metabolically active tissues, and this relationship is modulated by genetic variation in histone deacetylase (HDAC) enzymes that govern BHB’s epigenetic effects.

Mechanistic rationale

• BHB acts as an endogenous HDAC inhibitor, increasing acetylation of transcription factors such as FOXO3, which drives expression of autophagy genes (e.g., LC3B, BECN1) {[https://pmc.ncbi.nlm.nih.gov/articles/PMC12900492/]}.
• Individuals with rapid BHB elevation (>0.5 mM · h⁻¹ in the first 4 h of fasting) achieve higher intracellular HDAC inhibition sooner, leading to stronger FOXO3 acetylation and a larger autophagic flux, measurable as increased LC3B‑II/LC3B‑I ratio in peripheral blood mononuclear cells (PBMCs) {[https://pubmed.ncbi.nlm.nih.gov/41372565/]}.
• However, autophagy induction is tissue‑specific; liver shows robust responses whereas skeletal muscle shows minimal change {[https://www.intechopen.com/chapters/1143303]}. The proposed mechanism predicts that the BHB‑autophagy link will be strongest in liver‑derived signals (e.g., serum hepatokine FGF21) and PBMCs, but not in muscle‑derived markers.
• Genetic polymorphisms in HDAC6 or SIRT1 alter basal acetylation status and sensitivity to BHB‑mediated inhibition. Loss‑of‑function variants in HDAC6 already elevate FOXO3 acetylation, reducing the incremental effect of BHB; consequently, carriers may show a blunted autophagy response despite rapid ketone rise. Conversely, gain‑of‑function SIRT1 variants may enhance deacetylation of autophagy regulators, requiring higher BHB levels to achieve comparable flux.

Testable predictions

1. In a cohort undergoing a standardized 24‑hour fast, early BHB kinetics (area under the curve 0‑4 h) will positively correlate with LC3B‑II/LC3B‑I ratio in PBMCs (r > 0.4, p < 0.01).
2. The same BHB metric will correlate with liver‑specific autophagy readouts (e.g., serum FGF21 increase or MRI‑derived hepatic lipid droplet turnover) but not with muscle‑specific markers (e.g., myofibrillar LC3B staining from biopsy).
3. Stratification by HDAC6 rs11755690 (or SIRT1 rs7895833) genotypes will reveal interaction effects: the BHB‑autophagy slope will be significantly lower in HDAC6 loss‑of‑function carriers (p < 0.05 for interaction term).
4. Pharmacologic HDAC inhibition (e.g., low‑dose vorinostat) administered during fasting will equalize autophagy responses across genotypes, diminishing the genetic interaction.

Falsifiability If early BHB rise fails to predict autophagy flux in PBMCs or liver markers, or if genotype does not modify the BHB‑autophagy relationship, the hypothesis is refuted. Likewise, if HDAC inhibition does not abolish genotype‑dependent differences, the mechanistic link via HDAC inhibition is unsupported.

Implications Validating this hypothesis would enable a simple, inexpensive biomarker—early ketone kinetics—to personalize fasting duration and intensity, optimizing autophagic benefits while avoiding unnecessary prolonged restriction in non‑responders.